1. Field of the Invention
The present invention relates to a sewing machine drive apparatus which causes a sewing machine to generate holding force to keep a machine needle from moving and sticking into a cloth, or an object to be stitched, during a stop of the sewing machine.
2. Description of the Background Art
FIG. 17(a) shows the arrangement of a conventional sewing machine drive apparatus, wherein the numeral 1 indicates a sewing machine, 2 denotes drive means, e.g., a motor, which drives the sewing machine 1, 3 designates a sewing machine pulley, 4 represents a motor pulley, 5 indicates a belt which couples the pulleys 3 and 4, 6 designates a needle position detector fitted to the sewing machine 1 to detect the needle position of the sewing machine 1, 7 represents a detector which detects the position or velocity of the motor 2, 8 denotes a control box which controls the motor 2 to operate the sewing machine 1, 9 indicates a pedal operated by a worker to operate the sewing machine 1, and 10 represents a lever unit which converts the operation value of the pedal 9 into an electrical signal (for example, a velocity command value) and inputs the signal to the control box 8.
In the apparatus designed as described above, when the worker depresses the pedal 9, its depression value is converted into a velocity command value by the lever unit 10, the control box 8 operates the motor 2 at variable speed according to that velocity command value, and its drive force is transmitted to the motor pulley 4, the belt 5 and the sewing machine pulley 3 to operate the sewing machine 1. When, for example, the worker attempts to take out the cloth on completion of stitching after the operation of the sewing machine 1 through the control of the pedal 9, the sewing machine 1 must be stopped at a needle UP position to keep its needle from sticking into the cloth.
In some sewing machines 1 which utilize a presser bar plate spring, the spring is designed to be unloaded at a stop point in the needle UP position, shown at an 11 o'clock position of spindle rotation in FIG. 17(b). At other positions, the spring tends to bring the spindle back to the needle UP position, and is at a maximum compression at top dead center, shown as the 12 o'clock position. For example, because of the force of the spring (the force of motion is hereinafter referred to as the "machine load"), the sewing machine will rotate in the forward direction (counterclockwise) when the spindle is in a position after the 12 o'clock position, and will rotate in a reverse direction (clockwise) when the spindle is in a position prior to the 12 o'clock position. Accordingly, after a stop, this conventional sewing machine 1 continues to move and the needle position shifts. As a result, in an extreme case, the needle will stick into the cloth, and the cloth cannot be taken out.
For this reason, the conventional system may implement the position control of the motor 2 such that, during a stop of the sewing machine 1, the position control generates a torque for the motor 2 which is opposite to the direction in which the sewing machine 1 attempts to move, thereby providing holding force. Further, if there is a position deviation which exceeds a set value, the position control is designed to be cleared so that the worker can shift the needle position of the sewing machine 1 by hand in order to, for example, check the piercing position of the needle while such holding force control is being executed. These methods are described in details in, for example, Japanese Laid-Open Patent Publication No. SHO 62-106798.
FIG. 18 is a block arrangement diagram of a conventional sewing machine drive apparatus, wherein the numeral 11 indicates velocity command value changing unit, e.g., a selector switch, which is connected to point "a" in FIG. 18 to perform a variable-speed operation under the control of a velocity command from the lever unit 10 and is moved to a position control position at point "b" at the time of a stop to generate holding force during a stop (a position control during a stop is hereinafter referred to as a "soft brake"). 12 designates a velocity/torque conversion section which converts any velocity deviation, determined by the differences between the velocity command value and a velocity feedback value, into a torque command value. 13 represents a torque limiter which limits the torque command value to keep it from exceeding a set value. 14 denotes a driver consisting of power transistors, etc., to drive the motor 2 according to the torque command value. 15 indicates a detector, which may comprise an encoder within the motor 2, for detecting (conventionally by means of a light source, a light sensor, and rotary discs fitted to the motor shaft and provided with slits at predetermined locations) the angular value (travel) of the shaft of the motor 2. It is generally known that two sensors, which are disposed to electrically provide two phase pulse signals A and B with a phase difference of approximately 90.degree. , allow a rotation direction also to be detected. 16 represents a velocity detection section which detects velocity from such phased pulse signals A, B. The velocity detected thereby (hereinafter referred to as the "velocity feedback") is further converted into a positive or a negative value according to the rotation direction determined by using the method. 17 denotes a position detection section from which the direction of travel is output as a value whose polarity (positive or negative) is determined according to the rotation direction detected by the velocity detection section 16. 18 designates a position control section which exercises position control according to the movement value (hereinafter referred to as the "position feedback") from the position detection section 17. 20 indicates a position/velocity conversion section which converts the output of the position control section 18 into a velocity command value.
The operation of the sewing machine drive apparatus designed as described above will now be described in accordance with FIGS. 17(a) and (b), 18 and 19(a) and (b). FIGS. 19(a) and (b) show a relationship between position deviation and torque that is pertinent to the operation of the apparatus.
First, when the worker depresses the pedal 9, its depression value is converted into an electrical signal (velocity command value) by the lever unit 10. The velocity feedback from the velocity detection section 16 is subtracted from the electrical signal at the summing node 8a in the control box 8 and results in a velocity deviation value. The velocity deviation is converted into a torque command value by the velocity/torque conversion section 12. According to this torque command value, the driver 14 operates the motor 2. This is the typical movement performed during the operation of the sewing machine 1.
When the pedal 9 is set to a neutral position (a state wherein the worker does not depress the pedal), the velocity command value is zeroed, the motor 2 comes to a stop, and the sewing machine 1 is also brought to a stop. It is to be understood that some sewing machine drive apparatuses have an orientation function which allows the sewing machine to be positioned to a stop at its needle UP or DOWN position under the control of the signal from the needle position detector 6, but such sewing machines will not be described here.
When the sewing machine 1 comes to a stop, the selector switch 11 is moved from point "a" to the position control position at point "b" to provide soft brake processing. At the beginning of this change-over, as seen in FIGS. 19(a) and 19(b), the position deviation is zero and therefore torque is also zero. Shortly thereafter, because of the machine load, for example, the needle attempts to fall from top to bottom. Due to the coupling that exists, this also causes the motor 2 to move in a similar fashion, whereby a change occurs in the pulse signals A, B from the encoder 15. This change is converted by the position detection section 17 into a value with a sign related to the rotation direction, i.e., a position feedback signal which is a positive value for forward rotation or a negative value for reverse rotation, and is output to the position control section 18. The position control section 18 integrates this position feedback signal, converts it into a value representative of the position deviation from a home position immediately after the stop of the sewing machine 1, and outputs the result of the conversion. The position/velocity conversion section 20 inverts the sign of this output, converts the output into a velocity command value, and outputs the result of conversion to switch terminal "b". Thereafter, the motor 2 is driven to generate torque to return the sewing machine 1 to the home position as earlier described in the operation of the sewing machine.
A case where a position displacement has further occurred hereafter will now be described. This is a part wherein as described previously, the position deviation in excess of the set value is cleared so that the worker can shift the needle position of the sewing machine 1 by hand. When the position deviation, which is generated by the integration of the position feedback in the position control section 18 as described previously, has exceeded an optional first set value P, e.g., a travel of 5 degrees on the motor shaft, the position control section 18 clears the position deviation to zero. Accordingly, the torque is also zeroed (point h in FIG. 19(a)).
The relationships between the travel and torque at that time are shown in FIGS. 19(a) and 20(a). It should be noted that the value of +P shown in FIGS. 19(a) and 20(a) is a set value in excess of which the position deviation is cleared as described previously. Also, -T in FIGS. 19(b) and 20(b) indicates the holding force that exists at a time when the position deviation has exceeded the set value of +P and its value is a maximum torque value. In the meantime, a set value of -P and maximum holding force of +T, (which will not be described here, but are indicated by alternate long and short dash lines in the drawings) will exist when the sewing machine 1 is operated in the opposite direction. Region A shown in FIG. 19(a) indicates an interval from when the position deviation is zero until it is cleared, i.e., an interval from zero holding force to the maximum torque value.
As is indicated by the fact that the sewing machine 1 can be turned by hand, the machine load generally is smaller than the force required for moving the sewing machine 1 by hand, and moves the sewing machine 1 comparatively slowly. On the other hand, when the sewing machine 1 is moved by hand, the sewing machine 1 moves faster than when it is moved under the machine load.
On the basis of the explanation provided above, FIG. 20(b) can be seen to illustrate the case where the sewing machine 1 is moved fast at constant velocity and corresponds to the manual movement of the sewing machine 1. Similarly, FIG. 19(b) shows the case where the sewing machine 1 is moved slowly at constant velocity and corresponds to the movement of the sewing machine 1 under machine load. It should be noted that, since the machine load is smaller than the force required for manually moving the sewing machine 1 as described previously and the value of maximum torque -T of the holding force is actually set to balance them at a stop in region A. Unlike the case illustrated in FIG. 19(a), the region A is not exceeded (actually, the sewing machine stops at a position like point "j" with the holding force and the machine load balanced). FIG. 19(a) shows an operation wherein the region A has been exceeded; this may be referenced later for the purpose of a comparison between the conventional art and the embodiment of the present invention (a portion indicated by an alternate long and two short dashes line in FIG. 19).
The torque limiter 13 will now be described. FIG. 21 shows the characteristic of the torque limiter 13. In this drawing, the torque is limited to keep it from exceeding the maximum torque value (-T) of the holding force in the reverse direction when the velocity has a positive value (forward rotation), and the torque is limited to keep it from exceeding the maximum torque value (+T) of the holding force in the forward direction when the velocity has a negative value (reverse rotation).
The operation of the position control section 18 will now be described in accordance with a flowchart in FIG. 22. First, when the soft brake processing is initiated, the operation starts in step 50 and the position deviation is cleared in step 60. Then, in step 70, the value of the position feedback is added to the position deviation. Here, the operation will be described for a case where the sewing machine 1 has moved slowly, as shown in FIGS. 19(a) and 19(b). Since the value of the position deviation is small at first, the value of the position deviation is output in step 100 and the execution returns to step 70. As the execution passes step 70 several times, the position deviation value increases, a judgement is made in step 80 to branch to step 110, and the position deviation value is cleared by the processing of step 110. This is point "h" in FIG. 19(a). Hereafter, the same processing is repeated again. If, for example, the sewing machine 1 does not move, the position feedback value is zero and therefore the position deviation value does not change. Accordingly, the processing of step 110 is not performed either. It should be noted that when the selector switch 11 is changed over to point "a" to enter the operation mode, the processing in FIG. 22 is terminated forcibly and operation processing is then performed.
As described above, while the holding force which keeps the sewing machine 1 from being moved under machine load was provided in the conventional sewing machine drive apparatus, the position deviation was cleared if the travel, or the optional first set value P, was exceeded, whereby the worker could shift the needle position by hand.
It is to be understood that in such holding force control, typically, the operation of the soft brake can be controlled by a switch incorporated in the control box 8.
In the conventional sewing machine drive apparatus designed as described above, its holding force (torque) is controlled according to only the position deviation. Hence, whether the force for moving the sewing machine is machine load or worker power, the holding force (torque) was generated similarly in response to the position deviation. In addition to the power for moving the sewing machine by hand, therefore, the worker was further required to have the power to withstand the resistant force (holding force) from the motor. Moreover, since some sewing machines require an extremely large force to begin a movement, the maximum torque value of the holding force must be increased to keep the sewing machine from moving, whereby the resistant torque from the motor is increased. This poses a problem in the stitching world where there are many female workers.
Also, the worker is required to provide the power found by multiplying the holding force (torque), which is generally controlled to be constant on the shaft of the drive apparatus, by the pulley ratio of the sewing machine pulley diameter to the motor pulley diameter, because the worker actually applies the power to the sewing machine pulley to shift the needle position and the motor and the sewing machine are coupled by the motor pulley, the belt and the machine pulley as described previously. Hence, the worker is required to have the physical strength greater than that required when, for example, the motor pulley diameter was reduced for a low-speed sewing machine.
Furthermore, whether the sewing machine moves or not, a current flows during the soft brake processing exercised during a stop, whereby excitation noise is generated. Accordingly, even though the sewing machine is at a stop, the excitation noise of the motor continues to be generated, causing some workers to feel uncomfortable. For this reason, the soft brake had better be operated only when it is required, i.e., only when the sewing machine has moved under machine load after a stop. However, the worker had to make preparations and other work for the next stitching during the stop of the sewing machine and could not watch the sewing machine to turn on the soft brake switch when the sewing machine moved, whereby the soft brake had to be applied automatically.
In view of the aforementioned problems, a first object of the present invention is to provide a sewing machine drive apparatus which exercises control to generate a holding force during a stop of a sewing machine whereby a large holding force is provided under machine load; moreover, the holding force is kept from increasing so that a worker will not become fatigued when such worker must turn the sewing machine by hand.
A second object of the present invention is to provide a sewing machine drive apparatus which allows the sewing machine to be moved with predetermined force independently of the pulley ratio of the pulley diameter of the drive unit, such as a motor, to that of the sewing machine.